Monitoring of Cobalt and Cadmium in Daily Cosmetics Using Powder and Paper Optical Chemosensors.

Daily used cosmetics may contain high levels of heavy metals which are added to improve the quality and shine of cosmetics but represent a threat to human health. In this report, powder- and paper-based optical nanosensors using mesoporous silica nanospheres as carriers were designed for determination of Co2+ and Cd2+ in commonly used cosmetics. Powder optical chemosensors (POCs) were prepared via direct decoration of optical probes into a porous carrier. Paper-based chemosensors (PBCs) were designed via adsorbing the organic chromophore onto filter papers treated with mesoporous silica. POCs and PBCs were constructed with thick decoration of optical probes, leading to the formation of active surface centers for monitoring of Co2+ and Cd2+ in cosmetic products. The uniform structures of POCs and PBCs have resulted in selective sensing and low detection limits up to parts per billion, wide detection range determination, and fast response (on the order of seconds). Digital image colorimetric analysis (DICA) was used to quantify the color of PBCs and deduce the corresponding concentrations of Co2+ and Cd2+ using calibration curves. DICA data correlated well with that obtained from UV-vis spectrophotometry. The developed POCs and PBCs showed wide detection ranges of metal ions and a considerably low detection limit under optimal analysis conditions. The low limit of detection of Co2+ and Cd2+ ions using POCs was 6.7 × 10-9 and 3.5 × 10-9 M, respectively. To the best of our knowledge, this is the first time simple PBCs have been designed for monitoring Co2+ and Cd2+ with detection limits of 2.2 × 10-7 and 1.3 × 10-7 M. A limited amount of manufactured POCs (about 20 mg) were used for all measurements, and commercial filter paper treated with mesoporous nanosphere silica was used for sensing Co2+ and Cd2+ ions. The developed optical chemosensors had short regeneration times and exhibited high stability and surface functionality and are capable of monitoring Co2+ and Cd2+ in various cosmetic products.

Introduction

Commonly used cosmetics include shampoos, lotions, beauty creams and powders, lipsticks, toothpaste, and hair products. Some cosmetics contain toxic chemicals, such as toxic metals, formaldehyde, parabens, benzophenone, and phthalates, which could pose significant health risks to consumers.[1,2] Toxic ingredients in cosmetics have been correlated with disorders such as celiac,[3] breast cysts,[4] mitochondrial dysfunction,[5] and autism.[6] Skin exposure is one of the most important routes for the absorption of potentially toxic elements as most cosmetics are applied on the skin. Although the skin is a protective barrier, long-term use of cosmetics increases the rate of penetration of toxic ingredients, which results in irritation and allergies.[7] Skin lightening creams are among the most frequently used cosmetic products applied to the face to obtain a lighter skin tone or as an antispot treatment.[8] Toxic elements may be added to skin creams to improve the shine of the skin.[9] Adding pure metals to cream formulation is prohibited according to the European Union Cosmetics Regulation No 1223/2009.[10] Traces of toxic metals may be added to different cosmetics to improve their quality.[11] Heavy metals may enter the body in contaminated air, water, and food. Although heavy metal absorption is relatively low via the skin,[12] continuous and prolonged usage and application to a wide area leads to bioaccumulation of heavy metals.[13,14] High levels of metal ions are associated with formation of hydroxyl radicals which cause photo-oxidative damage.[15]

Cadmium is a deep yellow to orange pigment and mostly present in lipsticks, face powders, nail polish, soap, talcum powder, and also shaving creams. Cadmium is added to cosmetics as a color pigment.[16] The accumulation of Cd in the human body can lead to kidney disorders, brain damage, reproductive failure, and poisoning.[17] Additionally, cadmium may be present as impurities in cosmetic raw materials. Regardless, cadmium is exceptionally toxic and prohibited in cosmetics in the EU and U.S.[18]

Cobalt is commonly used in lipstick, eyeshadow, face painting, hair cream, shampoo, relaxers, and conditioners. Cobalt and its salts are also widely used as coloring agents in makeup and light-brown hair dyes.[19] Cobalt is a skin allergen responsible for allergic contact dermatitis.[20,21]

Various classical techniques were used to detect Co(II) and Cd(II) such as liquid–liquid,[22,23] cloud point extractions,[24] atomic absorption spectrometry,[25,26] solid-phase,[27,28] and inductively coupled plasma mass spectroscopy,[29] inductively coupled plasma atomic emission spectrometry (ICP-AES), ICP mass spectrometry (MS), chemiluminescence, and fluorescence spectroscopy analysis.[29] However, they have several disadvantages including complex procedures, the need for large infrastructure, highly trained staff, and high cost. Simple, sensitive, selective, and rapid detection of Co(II) and Cd(II) is therefore warranted. Recently, specific chromophores were attached to mesoporous nanomaterials for rapid and selective detection of different metal ions under optimal conditions.[30−33]

Mesoporous materials were used as carriers for chromophores to enhance the recognition and monitoring of heavy metals. The selected chromophore can be decorated on the surface of nanomaterials via chemical or physical interactions.[34−36] Digital image-based colorimetric analysis (DICA) was used for RGB color analysis of images captured by mobile phone cameras for fast and low-cost quantitative determinations of heavy metals captured on filter papers.[37,38]

In this study, we prepared stable optical chemosensors with two different approaches for ultrasensitive and selective monitoring of Cd2+ and Co2+ in cosmetic products. Mesoporous silica nanospheres with multichannel cages were designed to grow over filter paper to act as a platform for direct immobilization of chromophores specific for Cd2+ and Co2+. Spectrophotometric and DICA analyses were used for detection of Cd2+ and Co2+ in daily used cosmetic products (Scheme ). The designed chemosensors showed high stability and efficiency in visualization of Cd2+ and Co2+ with high sensitivity and selectivity at optimum conditions.

Scheme 1

Preparation of Solid-Based Chemosensors and Paper-Based Chemosensors for Sensing of Cd2+ and Co2+ Using Spectrophotometry and Digital Image-Based Analysis Techniques

Experimental Section

Chemicals

Milli-Q water was used for all of the tests. Cobalt chloride, cadmium chloride, CH3COONa, HCl, CH3COOH, and 1-(2-pyridylazo)-2-naphthol C15H11N3O were purchased from Sigma-Aldrich (St. Louis, MO). Acetone, cetyltrimethylammonium bromide (CTAB), diethyl ether, and tetraethyl orthosilicate (TEOS) were purchased from Aldrich Chemical Co Inc. Borax, sodium hydroxide, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and disodium hydrogen phosphate were purchased from Adwic-El Nasr Pharmaceutical Co (Cairo, Egypt).

Fabrication of Carrier and Chemosensors

Mesoporous silica nanospheres (MSNs) were prepared as described previously with some modifications.[39] The carriers were prepared in alkaline medium as powder and in neutral medium were grown over filter paper (Scheme ). Mesoporous cages were designed via ammonia-catalyzed hydrolysis of TEOS and specific surfactant, such as CTAB, in water, diethyl ether, and acetone solvent at pH 9 and room temperature. CTAB (0.5 g in 100 mL of deionized water) was stirred for 30 min. Then the solution was stirred for another 30 min after addition of 40 mL of acetone. To the previous mixture was added 20 mL of diethyl ether and stirred for 30 min. Then 2.5 mL of TEOS (precursor) was added to the mixture and stirred for 30 min. Finally, 1.5 mL of ammonium solution (25%) was added to adjust the alkaline medium, and a gel was obtained which was stirred for 1 day. After that, the silica/CTAB solid particles were collected by filtration and washed with deionized water and then allowed to stand at 80 °C for 24 h (Scheme A). In order to remove the cationic surfactant, a calcination process was done as follows: (1) increase the temperature from room temperature to 550 °C gradually for 4 h; (2) keep the temperature at 550 °C for 8 h to complete calcination process. Finally, the solid was left to cool gradually to room temperature. The organic probe was dissolved in 30 mL of ethanol and mixed with 0.5 g of MSN carrier. The solution was stirred for 4 h until saturation, and the obtained POCs were filtered and dried (Scheme ).

Scheme 2

Preparation of Powder Optical Chemosensors

In Scheme A, 0.5 g of CTAB was added to 100 mL of distilled water with stirring for 30 min in neutral medium. Then 10 mL of ethanol and 2.5 mL of TEOS were added to the mixture with stirring for 30 min. After that, 1.5 mL of sodium hydroxide was added with stirring for 2 h. Then the filter papers were dipped for 5 min for multiple times until saturation. The treated filter papers were dried at 50 °C for 6 h. The obtained filter papers were dipped in ethanolic solution of the selected chromophore to prepare PBCs, as shown in Scheme B. The prepared PBCs were cut into suitable size and inserted into kits to use in monitoring of toxic metals.

Scheme 3

Preparation of PBCs: (A) Growth of Mesoporous Nanosphere over Filter Paper in Neutral Medium and (B) Treated Filter Paper Chemosensors Directly Dipped into Ethanolic Solution of Chromophore and Cut into Suitable Sizes for Sensing of Cd2+ and Co2+

Analysis of Toxic Metal Ions

Stock solutions of Co2+ and Cd2+ (200 ppm) were prepared by dissolving CoCl2 and CdCl2 in Milli-Q water in a 100.0 mL volumetric flask. In the selectivity studies, all of the metal ions were prepared in Milli-Q water using the same procedure. The POC-suspended solution was placed in a quartz optical cell with a 1 cm optical path length, and then the appropriate amount of the metal ion solution was added. UV–vis spectra were recorded within a few seconds after addition of the various analytes without any vigorous stirring or shaking. In heterogeneous sensing assays, metal ion concentrations were adjusted at different pH values. This mixture was directly added to 20 mg of the POCs at constant volume (10 cm3) with shaking at room temperature. A blank solution was also prepared, following the same procedure for comparison. After equilibration response time (Rt), the chemosensor solution was sonicated for 1 min. The sonicated samples were analyzed by spectrophotometric methods. The colorimetric determination and visual detection of analyzed ions using chemosensors was carried out over a wide range of pH solutions. About 20 mg of the POCs and PBCs was added to several aqueous solutions (with a total volume of 10 mL) containing specific concentrations of analyte and adjusted increasing pH values (within 2–12 range). Then these solutions were sonicated for 5 s. After equilibration, a prominent color change and the signal saturation of the POCs’ absorbance spectra were achieved.

Results and Discussion

Structure Morphology of POCs and PBCs

The mesoporous carriers, the POCs, and the complex diffraction peaks were examined using X-ray diffraction (XRD) patterns (Figure A). Our findings revealed well-defined and broad diffraction peaks at 2θ = 23.0°, which confirm the mesostructured morphology of the prepared carrier.[37] The dressing layer of the organic chromophore onto the mesoporous cages of the ball-like structure carrier give the same diffraction peaks of the unloaded silica carrier. Therefore, the high stability evidences the mesoporous structure under decoration and complexation conditions. Our data indicate a dramatic decrease in the intensity of MSNs and POCs, which confirm the filling of mesoporous cages before and after complexation with targeted ions. The small-angle XRD profile of carrier and POCs showed a single diffraction peak with a d-spacing ratio of 3.79 nm (Figure S1), demonstrating the formation of mesopore designs. The low resolution of the high intense peak at 2.0 ≤ 2θ ≥ 2.5 indicates the formation of mesopores with spherical structures (as evidenced from scanning electron microscopy images).[37] To investigate the diffraction peaks of treated filter paper with mesoporous silica and PBCs, XRD patterns were examined, as shown in Figure B. The treated filter paper with MSNs, PBCs, and PBCs+M2+ showed four diffraction peaks that can be assigned to (110), (110), (200), and weak (004) planes. The obtained data are characteristic for the structure of cellulose.[38] A significant decrease in the treated paper with MSN diffraction intensities after decoration with a chromophore was observed. Our results suggested the interaction between the hydrogen bond of the cellulosic polymer chains with MSNs and the chromophore.[39]

Figure 1

Wide-angle XRD diffraction patterns of (A) treated filter paper with MSNs, treated filter paper with chromophore, and complex. Wide-angel XRD diffraction patterns of (B) MSN as powder, powder-based optical chemosensors, and after complexation of POCs with metal ions.

The morphology of the free and treated filter paper was investigated using field emission scanning electron microscopy (FESEM), as shown in Figure A,B. Our results showed formation of a thin film layer covering the cellulose fibers chain pores by MSNs. Our obtained images confirm the blocking of cellulose fiber cages after decoration with the chromophore to design PBCs (Figure C,D). Moreover, the mesoporous carrier and POC structure morphology were also examined (Figure E–H). The obtained spherical shape with a ball-like structural morphology of the mesoporous carrier agreed with XRD pattern results and confirmed the successful fabrication of MSNs and POCs. Furthermore, the Figure G,H further clarifies the mesopore treatment after a direct decoration process and complexation into the nanosphere silica of POCs.

Figure 2

FESEM images of free filter paper (A), filter paper with MSNs (B), MSN filter paper with chromophore (C), and interaction of carrier with chromophore and complex on the treated filter paper (D). (E–H) Higher-magnification SEM of mesoporous silica nanosphere powder (E,F), powder-based optical chemosensors after immobilization process (G), and interaction between POCs with metal ions.

To examine the surface area and pore size of the fabricated POCs, PBCs, and its complexes, the N2 isotherms and nonlocal density functional theory (NLDFT) studies were investigated.[40−42] In Figure A, the untreated and the treated filter papers with mesoporous nanosphere isotherms were examined. Our findings investigated that the filter paper, treated paper with MSNs, PBCs, and complex have the same type isotherm (type III). The surface area of the untreated filter paper (1.95m2 g–1) was enhanced after treatment with MSNs (5.79 m2 g–1), which confirmed the formation of MSNs over the filter paper. However, a sharp decrease in the surface area of PBCs (1.31 m2g–1) and complex (0.919 m2 g–1) indicated the successful loading of a chromophore on the treated filter paper. DFT studies demonstrated the existence of multiple pore diameters of the treated filter paper with MSNs. The pore diameter of the treated filter paper was significantly reduced after an immobilization process, which confirmed the formation of PBCs for sensing of toxic metal ions. In Figure C, our results show that the carrier, POCs, and POCs+M2+ have the same type of isotherm (type IV), which confirms the high stability of the prepared mesoporous carrier. A considerable decrease in the carrier isotherm confirms the surface area reduction of the prepared carrier. The POCs and POCs+M2+ surface areas were found to be 346.06 and 339.44 m2 g–1, which are lower than that of the carrier (475.68 m2 g–1). In Figure D, the NLDFT data clarify the formation of mesoporous structures (pore size <2 nm). The obtained results confirm the decoration of the chromophore and complex around a nanorod and inside the mesoporous cages.

Figure 3

(A) N2 adsorption/desorption isotherms and (B) NLDFT pore size of treated filter paper with MSNs, treated filter paper with chromophore, and complex. (C) N2 adsorption/desorption isotherms and (D) NLDFT pore size of the nanosphere carrier, POC chemosensors, and POCs+M2+ complex.

UV–Vis Determination of Co2+ and Cd2+

To evaluate the performance of Co2+ and Cd2+ optical chemosensors as POCs or PBCs, the working parameters were investigated. These variables have a significant impact on color dispersion uniformity and intensity at extremely low metal ion levels. Standard solutions of both ions were generated using buffer solution at various pH ranges. Furthermore, the absorbance was measured (Figure ) to determine the best pH value for sensing of Co2+ and Cd2+ using POC and PBC kits. Our finding determined that the most suitable pH for sensing cobalt ions was pH 7 in the presence of both POC and PBC kits, as shown in Figure A. In Figure B, our findings exposed that the high absorbance intensity of POCs for sensing of sensing of Cd2+ was reported at pH 9. On the other hand, pH 8 was chosen as the best for detecting Cd2+ using PBCs.

Figure 4

Effect of pH on response of (A) cobalt paper and powder optical chemosensors and (B) cadmium paper and powder optical chemosensors for detection of 2 ppm of Co2+ and Cd2+, respectively.

Selectivity of POCs for Sensing of Co2+ and Cd2+

To assess the effect of interfering ions on sensing of Co2+ and Cd2+, a series of mono-, di-, and trivalent cations such as Na+, K+, Cs+, Mn2+, Ca2+, Mg2+, Ni2+, Pb2+, Ba2+, Zn2+, Cu2+, Hg2+, Al3+, Cr3+, and Fe3+ were tested under optimal sensing conditions (2 mg/L of Co2+ and Cd2+, 5 mg of POCs, pH 7 for Co2+ and pH 9 for Cd2+, and 298 K). Spectrophotometry was utilized to investigate the selectivity of the POCs for detection of Co2+ and Cd2+ in the presence of other cations. The absorbance spectra were measured in the presence and in the absence of 1 ppm of Cd2+ or Co2+ (Figure ). Figure A,B shows that common interfering ions had no considerable enhancement on the absorbance spectra of POCs. However, POCs showed great absorbance intensities in the presence of Co2+ at pH 7 and Cd2+ at pH 9, which confirm the high selectivity of the prepared POCs toward Co2+ and Cd2+. The tolerance for common ions during detection of Co2+ (pH 7) or Cd2+ (pH 9) using POCs is presented in Table S1. The results confirm the high selectivity of POCs toward Co2+ and Cd2+. Moreover, the high concentration of common interfering ions can be eliminated by masking with EDTA (0.01 M) and Co2+ and Cd2+. The addition of formaldehyde could be used to demask the targeted ions.

Figure 5

Effect of common interfering cations on absorbance spectra of (A) optical chemosensors in the presence and absence of Cd2+ (2 ppm) and (B) optical chemosensors in the presence and absence of Co2+ (2 ppm).

Sensing of Co2+ and Cd2+

In the colorimetric-based analysis for sensing Co2+ and Cd2+, the absorption spectra of POC and PBC kits were investigated in the presence of increasing concentrations of Cd2+ and Co2+. In Figure A, our results demonstrate enhancement in absorption spectra at λmax = 556 nm with increasing the Cd2+ levels. Moreover, the existence of isosbestic point at λmax = 524.5 nm and absorbance intensity enhancement were attributed to complex formation and the charge transfer. The calibration curve of POCs showed a linear correlation with ultratrace concentration of Cd2+, as shown in Figure B. The linear correlation demonstrates that Cd2+ may be detected quantitatively with exceptional sensitivity. Moreover, our simple process allows for recognition of Cd2+ from 0.002 to 2.2 μM via change in the POCs’ color from yellow to deep red. The of limit of detection (LD) and limit of quantification (LQ) of sensing Cd2+ using POCs were estimated to be 3.5 × 10–9 and 11.55 × 10–9 M, respectively, which is lower than the allowed limit in water.

Figure 6

(A) Absorption spectra of POCs upon titration with Cd2+ under optimum sensing parameters. (B) Calibration curves for POCs measured for the Cd2+; linear fit line is inserted in the linear concentration range with various concentrations of Cd2+. (C) PBC absorption spectra upon titration with Cd2+ under optimum conditions. (D) Calibration curves for PBC chemosensors measured for the Cd2+; linear fit line is inserted in the linear concentration range with various concentrations of Cd2+.

In paper-based Cd2+ ion chemosensors, the absorption spectra of PBC kits were investigated in the presence of increasing Cd2+ concentrations in aqueous solution. In Figure C, we show our findings on the enhancement in absorption spectra at λmax = 567 nm with increasing the Cd2+ levels. The calibration curve of PBCs showed a linear correlation with low levels of Cd2+, as shown in (Figure D). Moreover, our simple procedure allows for monitoring of Cd2+ from 0.002 to 8.8 μM via change in the PBC color. The low LD and LQ of sensing Cd2+ using PBCs were estimated to be 1.3 × 10–7 and 4.29 × 10–7 M, respectively (Table ). Our result showed that the fabricated POCs and PBCs had a greater recognition of Cd2+ compared with the reported chemosensors (Table ).

Table 1

Analytical Parameters for Detection of Cd2+ and Co2+ Using Powder Optical Chemosensors (POCs) and Paper-Based Chemosensors (PBCs)a

	Cd2+		Co2+
parameters	POCs	PBCs	POCs	PBCs
LD (M)	3.5 × 10–9 M (0.393 ppb)	1.3 × 10–7 M (16.862 ppb)	6.7 × 10–9 M (0.3946 ppb)	2.2 × 10–7 M (12.958 ppb)
DR (M)	(0.002–2.2) × 10–6 M	(0.002–8.8) × 10–6 M	(0.002–3.5) × 10–6 M	(0.002–2.2) × 10–6 M
LQ (M)	11.55 × 10–9 M	4.29 × 10–7 M	22.1 × 10–9 M	7.26 × 10–7 M
R2	0.982	0.989	0.989	0.991

Limit of detection (LD), detection range (DR), limit of quantitation (LQ).

Table 2

Comparing Detection Limits of Different Cd2+ Optical Chemosensors

reagent/sensor	detection limit (mol/L)	ref
cubic mesocage sensors	3.07 × 10–8	(43)
chemosensors for Cd2+	19.96 × 10–8	(44)
dithiazone TiO2 sensor	1.56 × 10–8	(45)
aluminosilica optical sensor (ASOS)	2.464 × 10–9	(46)
green AuNP probe	3 × 10–8	(47)
dithiocarbamate-functionalized gold nanoparticles	6.29 × 10–8	(48)
POCs	3.5 × 10–9 M	current study
PBCs	1.3 × 10–7 M	current study

For sensing cobalt ions using POCs, the absorption spectra were investigated in the presence of increasing Co2+ levels. In Figure A, our results demonstrate increasing absorption spectra at λmax = 570 and 630 nm with increasing Co2+ concentration. Furthermore, the appearance of an isosbestic point at λmax = 524.5 nm and absorbance intensity enhancement were accredited to the complex formation and charge transfer. The POC calibration curve for sensing Co2+ showed a linear correlation with a low level of Co2+, as shown in Figure B. Our results show that the Co2+ ion can be evaluated with ultrasensitivity. Moreover, our simple procedure allows for detection of Co2+ from 0.002 to 3.5 μM via a change in the POCs’ color from yellow to green (Scheme ). The LD and LQ for detecting of Co2+ ion using POCs were estimated to be 6.7 × 10–9 and 22.1 × 10–9 M, respectively.

Figure 7

(A) Absorption spectra of POCs upon titration with Co2+ under optimum working parameters. (B) Calibration curves for POC chemosensors measured for the Co2+; linear fit line is inserted in the linear concentration range with various concentrations of Co2+. (C) PBC absorption spectra upon titration with Co2+ under optimum conditions. (D) PBC calibration curves measured for the Co2+; linear fit line is inserted in the linear concentration range with different concentrations of Co2+.

In Co2+ PBCs, the kit’s absorption spectra were demonstrated in the presence of elevated Co2+ levels in aqueous solution. In Figure C, the absorption spectra enhancement of PBCs at λmax = 576 nm and λmax = 631 nm with increasing Co2+ concentration was studied. Moreover, the PBC’s calibration curve showed a linear correlation with low Co2+ level (Figure D). The LD and LQ for sensing of Co2+ using PBC kits were estimated to be 2.2 × 10–7 and 7.26 × 10–7 M, respectively (Table ). Our data show that the fabricated chemosensors had an excellent monitoring compared with the reported optical sensors (Table ).

Table 3

Comparing Detection Limits of Different Co2+ Optical Chemosensors

sensor	detection limit (mol/L)	ref
azo-HNTA probe	0.77 × 10–8	(49)
CpAD probe	6.6 × 10–9	(50)
chemosensor for cobalt	1.8 × 10–6	(51)
aluminosilica optical sensor (ASOS)	2.8 × 10–9	(52)
chemosensor (HL) based on coumarin	3.1 × 10–7	(53)
POCs	6.7 × 10–9	current study
PBCs	2.2 × 10–7	current study

Sensing Mechanism of POCs

Mesoporous nanospheres coated with organic probes were used to sense the presence of Cd2+ or Co2+ under specific sensing conditions. The availability of oxygen and azo-nitrogen for complexation of Cd2+ or Co2+ with POCs leads to the development of two coordination spheres and a stable complex. The stoichiometric ratio between Cd(II) or Co(II) to probe at specific pH is assumed to be 1:2, as shown in Scheme . The results demonstrate the enhancement in absorption spectra with increasing Co2+ levels (Figure A) or Cd2+ levels (Figure A). Furthermore, the existence of an isosbestic point was attributed to complex formation and the charge transfer. This approach allows for naked-eye detection of ultratrace concentration of metal ions without the need for complex classical approaches.

Scheme 4

Schematic Mechanism of the Possible Interactions of the POCs with Cd2+ and Co2+ under Optimal Sensing Conditions

Signal sensing responses of POCs to Cd2+ at pH 9 and Co2+ at pH 7 with the formation of the red [Cd2+-probe] complex and green [Co2+-probe] complex via the heteroatom of the optical probe.

Digital Image-Based Colorimetric Analysis

The changes in RGB intensity values (IR, IG, and IB) of PBC kits were investigated using captured images in relation to Co2+ and Cd2+ concentrations. Adobe Photoshop CS6 was used to evaluate the captured images of PBC kits that were provided by the histogram tool. In Figure A, there is significant enhancement on green IG intensity with increasing Cd2+ concentrations. Our findings showed a remarkable agreement with the obtained results using a spectrophotometric technique. The color intensity can likewise be directly related to the absorbance with different concentration using eq S2 (Figure B). The linear correlation between the green color absorbance and the Cd2+ ion concentration was investigated, as shown in Figure C.

Figure 8

(A) Relationship between [Cd2+] and the RGB intensities using the histogram tool. (B) Relationship between [Cd2+] and the calculated absorbances from RGB of the images taken with a mobile camera. (C) Linear correlation between absorbance of green color and [Cd2+].

The change in RGB intensity values of PBC kits for sensing of Co2+ was investigated using captured images in relation to Co(II) levels (Figure ). In Figure A, the result shows that the red color intensity decreases with increasing Co2+ ion concentrations. The red color intensity is directly related to the absorbance with different Co2+ concentration (Figure B). The linear correlation between the red color absorbance and the Co2+ ion concentration is shown in Figure C. The obtained data confirmed the applicability of DICA in sensing of low levels of Co2+ and Cd2+. The results show a significant agreement compared with spectrophotometric method, and good evidence explained that DICA can be used as a portable, inexpensive, and semiquantitative analysis tool for recognition of Co2+ and Cd2+. Consequently, DICA appears to be an effective method for the quantification of Co2+ and Cd2+ concentrations.

Figure 9

(A) Relationship between [Co2+] and the RGB intensities using a histogram tool to analyze the digital images taken by mobile camera. (B) Relationship between [Co2+] and the calculated absorbances of RGB for the images taken with a mobile camera. (C) Linear correlation between absorbance of red color and [Co2+].

Applicability of POC Nanospheres

To detect Co2+ and Cd2+ concentrations in real samples, daily used cosmetics (chosen from various locations) were examined using POC nanospheres. The samples are first dried at 80 °C for 24 h and then digested with nitric acid. Approximately, 1.0 g of each sample (in triplicate) was placed in a 50 mL beaker containing 20 mL of HNO3 and heated to 90 °C by gently raising the temperature on a hot plate. Once digested, the samples were cooled and filtered using Whatman filter paper and then diluted with deionized water to a final volume of 100 mL. Digested samples were placed in clear glass vials for further investigation. Co2+ and Cd2+ concentrations in sample solutions were determined using an ICP-AES. The working standards were freshly prepared on the day of analysis using standard stock solutions (200 mg/L) of the metals. All measurements were done in triplicate. The concentration of Co2+ in sample 1 was found to be between 1.22 and 1.52 μg/g, while the concentration of Cd2+ in sample 2 was found to be between 0.89 and 0.96 μg/g. The mean ± SD concentration of Co2+ was 1.4 ± 0.131 μg/g, and for Cd2+, it was 0.92 ± 0.028 μg/g. To test the presence of Co2+ and Cd2+ using the developed probe, at optimum monitoring conditions, 20 mg of POC nanospheres was added to the prepared cosmetic sample spiked with different concentrations of Cd2+ or Co2+ ions (Table ) at pH 7.0 (for detection of Co2+) or pH 9.0 (for detection of Cd2+). The color of the chemosensors changed immediately in the presence of toxic metal ions. Measurements of Co2+ and Cd2+ in cosmetic samples were repeated in triplicate, and results were assessed using UV–vis spectrophotometric techniques (Table ).

Table 4

Determination of Co2+ and Cd2+ Concentrations in Daily Used Cosmetics Using POCs and PBCsa

		POCs/spectrophotometry			PBCs/DICA
samples	metal ion (μg/L) (ppb)	recovery (μg/L)	E (%)	error (%)	recovery (μg/L)	E (%)	error (%)
sample 1*	30	29.16	97.2	2.8	30.72	102.4	2.4
	60	60.84	101.4	1.4	61.47	102.45	2.45
	100	98.32	98.32	1.68	98.73	98.73	1.27
sample 2**	20	20.54	102.7	2.7	19.67	98.35	1.65
	50	51.13	102.26	2.26	49.17	98.34	1.66
	100	98.06	98.06	1.94	101.74	101.74	1.74

POCs: Powder optical chemosensors, PBCs: Paper based chemosensors, DICA: Digital image-based analysis, E%: efficiency percentage; “*” is extracted cosmetic sample spiked with different concentration of Co2+ ions; “**” is extracted cosmetic sample spiked with different concentration of Cd2+ ions.

Conclusions

The POCs and PBCs based on nanosphere mesoporous carriers have a significant abundance for monitoring and Co2+ and Cd2+ concentration in daily used cosmetics. The POC and PBC kits were fabricated via two dissimilar approaches for ultrasensitive and ultraselective monitoring of Cd2+ and Co2+ in cosmetic products. The successful engineering of mesoporous silica and growth over filter paper were explained. The POCs and PBCs were considered via direct decoration and a dipping process of organic chromophore into mesoporous powder and the treated filter paper with MSNs. The designed POCs and PBCs based on the nanosphere show high stability and efficiency in visualization of Cd2+ and Co2+ with high sensitivity and selectivity at optimum conditions. The naked-eye color progress of POC and PBC complexes gave an additional simultaneous colorimetric detection of Cd2+ and Co2+ without using traditional methods. Moreover, POC and PBC kits can be used multiple times (using simple regeneration procedures) and have long-term stability and high detection efficiency. To the best of our knowledge, this is the first study that employs PBC kits for sensing Cd2+ and Co2+ using spectrophotometry and DICA techniques. Our findings suggest that the fabricated POC and PBC kits have great potential for visualizing the presence of Cd2+ and Co2+ in daily used cosmetics.